1. Field of the Invention
The present invention relates to oriented multilayer films. More particularly, the invention pertains to coextruded or laminated films having at least one layer of a fluoropolymer such as poly(chlorotrifluoro ethylene) (PCTFE) homopolymer or copolymer, a layer of a polyolefin homopolymer or polyolefin containing copolymer and an intermediate adhesive layer of a polyolefin having at least one functional moiety of an unsaturated carboxylic acid and/or anhydride thereof
2. Description of the Prior Art
It is well known in the art to produce oriented polymeric films. See, e.g. U.S. Pat. No. 4,011,874. However, such films tend to expand in the direction perpendicular to the direction of stretching.
It is also known in the art to produce single layer and multilayer fluoropolymer films. See, e.g. U.S. Pat. Nos. 4,677,017; 4,659,625 and 5,139,878, all of which are incorporated herein by reference. However, fluoropolymers are difficult to orient due to their unique crystallization properties. More particularly, PCTFE is exceptionally difficult to orient due to its extremely fast crystallization rate and thermally induced self-orientation. Its fast crystallization rate produces a highly crystalline structure that hinders orientation and actually prevents further orientation beyond a certain point. Its thermally induced self-orientation results in a film which, upon unconstrained heating, self extends in the machine or longitudinally stretched direction and shrinks in the transverse direction.
Most earlier attempts to stretch PCTFE films have failed either due to its high degree of film crystallinity, nonuniform crystallinity, self-orientation or a combination of these factors. Prior art studies of the orientation of PCTFE homopolymer report a limit of a three to four times orientation or stretch ratio in either the machine direction (MD) or transverse direction (TD). For example, U.S. Pat. No. 4,544,721 describes a substantially amorphous chlorotrifluoroethylene polymer monolayer film which is oriented at least 2.5 times its original length, but no more than five times in the MD. It also disclosed therein that attempts to stretch crystalline PCTFE result in films that contain holes or tears, or which are uneven in thickness. Other known attempts to stretch PCTFE homopolymer more than five times its unstretched length result in film fibrilation and ultimate breakage. See, e.g. U.S. Pat. No. 4,510,301 (orients film containing a copolymer of ethylene and chlorotrifluoroethylene).
It would be desirable to produce a much more highly oriented, dimensionally stable fluoropolymer film since as the higher the degree of attainable orientation is increased, the properties of mechanical strength, toughness, and water vapor barrier capability are significantly improved without increasing the film gauge. It would also be desirable to produce a multilayered film structure which is dimensionally stable and uniform across its entire width.